Printed circuit boards are produced using photoresists. According to the dry film photoresist system disclosed in Japanese Patent Publication No. 25231/70, a photoresist in the form of film is laminated onto a copper-clad laminate (base), the photoresist is exposed to the activating light through a mask having the wiring pattern, and the unexposed parts are dissolved using an adequate developing solution, whereby a cured image is obtained on the base. The image is used as the resist and the copper not covered by the resist is etched, plated, or anodized. Thus, there is obtained a printed circuit board.
The dry film photoresist is not desirable because its adhesion to a metal surface is weaker than that of the solution type photoresist. This weak adhesion causes an unfavorable phenomenon during etching or plating. For example, when the base is sprayed with an etching solution or dipped in a plating solution, the solution infiltrates into the interface between the resist and the base, causing the resist to separate from the base. This leads to underetching or underplating, which, in turn, causes the image to become blurred or to disappear. If the adhesion problem is substantial, the desired pattern is not obtained and the base is wasted.
In the production of printed circuit boards by the use of a dry film resist, the resist pattern is closely attached to the base over the entire surface in some cases. In other cases where the base has through-holes, the resist pattern is not attached over the entire surface. (The through-holes are holes made through the base, with the inside surface thereof coated with a metal layer such as a copper layer, and they electrically connect the printed circuits formed on both sides of the base.) In such cases, it is necessary to stretch the resist film over the opening of the hole, so that the metal layer on the inside of the hole is protected from etching. The resist film thus formed sticks to the base at a very small area around the opening of the hole, and the film covering the opening of the hole is not supported. Yet, the resist film is required to have sufficient peel resistance against the etching spray.
In order to improve the peel resistance, pretreatment of the metal surface was proposed. (Japanese Patent Publication No. 5292/79 (U.S. Pat. No. 3,873,316) and Japanese Patent Laid-Open Nos. 64919/76 and 64920/76 (U.S. Pat. No. 4,233,395))
Several other methods for improving the adhesion by adding a variety of compounds to the photosensitive resin layer were also proposed. (Japanese Patent Publication Nos. 9177/75 (U.S. Pat. No. 3,622,334), 5292/79 (U.S. Pat. No. 3,873,316), 22481/80, 46053/82, 46054/82, 21697/82 and 40500/82, and Japanese Patent Laid-Open Nos. 64919/76, 64920/76, 63087/75, 2724/77, 702/78, 124541/78, 124594/78, 133585/79, 133586/79, 65947/80, 11904/81, 75642/81, 67844/81, 99202/81, 100803/81, 60327/82 and 62047/82)
The surface treatment in the former method requires an additional step. Therefore, the latter method is superior in this respect; but it is not desirable because the copper surface which is exposed after development turns red, and this discoloration adversely affects the subsequent etching, plating, or soldering. Moreover, some additives interfere with the photopolymerization reaction and have low miscibility with the photopolymerizable resin composition.